Prefetch frames works only at opening blend-file?
Changing the cache limit size or the cache type (use disk cache), Ctrl+R (or Alt+R) does not reinitialize the prefetch cache. The only work-around so far is saving the file and re-open it. At that moment the prefetch kicks in, the little orange stripe at the bottom keeps growing from the play head and (after a while) the preview runs smoothly again. This keeps on working until you change something in the timeline. Then the orange cache bar dims out and the performance drops to a much lower frame rate. Apparently, this is also the case in version 2.83 and 2.90.